The HipBA toxin-antitoxin system adopts an unusual three-com-ponent regulatory mechanism.

Type II toxin-antitoxin (TA) systems encode two proteins: a toxin that inhibits cell growth and an antitoxin that neutralizes the toxin by direct inter-molecular protein-protein inter-actions. The bacterial HipBA TA system is implicated in persister formation. The HipBA TA system consists of a HipB antitoxin and a HipA toxin, the latter of which is split into two fragments, and here we investigate this novel three-com-ponent regulatory HipBA system. Structural and functional analysis revealed that HipA corresponds to the N-ter-minal part of HipA from other bacteria and toxic HipA is inactivated by HipA, not HipB. This study will be helpful in understanding the detailed regulatory mechanism of the HipBA system, as well as why it is constructed as a three-com-ponent system.